Hydraulic units of this type are, among others, made use of in brake slip control systems. In these cases, the valve block also has a piston pump which is driven by an electric motor flanged to the valve block. As an example, the hydraulic unit described in U.S. Pat. No. 50,227,171 is referred to. In the version described in this patent, the low-pressure accumulator is integrated completely in the valve block which means that the accumulator piston is guided in a blind-end bore in the valve block which is closed by a plate screwed to the corresponding lateral surface of the valve block. The accumulator spring is arranged between the accumulator piston and the plate. The depth of the blind-end bore is determined by the axial extension of the accumulator piston and the accumulator spring length which, in turn, corresponds to the maximum absorption volume of the low-pressure accumulator that is required to be provided for the respective system. Among other things, care should be taken that, even with a maximally filled low-pressure accumulator, the accumulator pressure must not reach an excessive value. For this reason, for example, the accumulator spring must not be compressed to its coil bound length. Taking these conditions into account, there is a space requirement for the low-pressure accumulator which is significantly larger than the maximum volume absorption. This is in contrast to the requirement of minimizing the valve block under weight and space aspects.
European patent application No. 0 770 530 describes a hydraulic unit which has a housing cap on an end surface of a valve block which covers the low-pressure accumulator provided in the end surface and a plurality of valves. The low-pressure accumulator is plunged entirely into a bore in the valve block and furnished with a cover that extends between the housing cap and the end surface of the valve block into the hollow space of the housing cap.
Generic German patent application No. 42 34 013 discloses a hydraulic unit having a low-pressure accumulator which is closed with a cap-shaped add-on piece on one lateral surface of the valve block. Several electromagnetically operable valves are arranged on an end surface of the valve block which extends rectangularly to the lateral surface, and a bore extends through the valve block to provide a cable duct for connecting an electric motor. The positioning of the low-pressure accumulator and the valves on different housing surfaces and the necessity of making a through-bore for the cable duct limit the possibility of reducing the dimensions of the unit and simplifying its manufacture.
Therefore, an object of the present invention is to render the hydraulic unit of the type disclosed, by simple means, still smaller and lighter in weight than has been provided by the previously suggested solution.
Favorably, the accumulator piston is guided in the add-on piece, and part of the housing of the low-pressure accumulator, which is formed by the valve block itself, radially encloses a volume that corresponds roughly to the maximum accumulator volume of the low-pressure accumulator.
An accordingly configured low-pressure accumulator is especially appropriate when the number of valves integrated in the valve block and required for pressure control is small. It is arranged for in the state of the art to position the valve actuators side by side on a base of the valve block. In a system with a reduced number of valves, there remains an area into which the add-on piece can project in the space beside the valve block where the actuators are arranged.
Usually, the actuators are covered by a housing cap on the valve block which, among others, also accommodates the electronic control unit. In the proposed arrangement, the low-pressure accumulator is accommodated in the space enclosed by the housing cap and, thus, protected against outer influences. Among others, this permits designing the add-on piece like a bowl deepdrawn from sheet metal which is inserted into a flat blind-end bore in the valve block. The add-on piece has an outwardly directed flange-like beaded rim which, additionally, is used as an abutment surface for a sealing ring disposed in a step-like expansion of the blind-end bore.
The accumulator piston is also deepdrawn from metal in a bowl-type configuration, and the add-on piece and the accumulator piston, aligned with their open sides towards each other, are inserted into each other. The space enclosed by the accumulator piston and the add-on piece accommodates the accumulator spring which is supported on the bottom of the accumulator piston, on the one hand, and on the bottom of the add-on piece, on the other hand.
The accumulator chamber which is enclosed by the accumulator piston in the blind-end bore in the valve block, by way of a straight bore, is in connection with a transverse bore in the valve block. The transverse bore in which a pump piston is guided opens into a damping chamber. Inserted into the mouth of the connecting bore into the accumulator chamber is a non-return valve which represents the pump suction valve for the pump. The pump pressure valve of this pump is also configured as a non-return valve and inserted into the mouth of the transverse bore into the damping chamber.
In parallel to the transverse bore, a connecting bore extends between the damping chamber and a pressure fluid channel in the valve block and is narrowed in an orifice-type fashion directly in front of where it opens into the channel.
The electric motor projects from a lateral surface of the valve block so that a connecting cable, starting from the bearing plate of the motor housing, can be passed by the side of the valve block and guided to the electronic controller that is arranged within the housing cap.
To protect the cable connection against outer influences, the housing cap has an extension which passes by the valve block up to the bearing plate. Together with the side wall of the valve block, the extension forms a channel through which the electrical connecting cable is conducted.